Some electric motors operate with a variable frequency drive (VFD) system that adjusts the rotational speed of the motor by varying a frequency of an alternating current (AC) electrical power signal that drives the motor. During operation of the motor, the AC power signal can induce an electrical charge on the surface of the motor drive shaft. The electrical charge can build to a level sufficient to enable an arc from the shaft to another component in or around the motor. The drive shaft bearing is one component in an electric motor that can experience arcing from the drive shaft because the bearing is close to the drive shaft and often provides a path of least resistance for the electric charge. The arcing from the drive shaft damages components in the bearing and may require premature replacement of the bearing.
In order to reduce or eliminate arcing through the motor bearing, some motors use ground rings. A ground ring is an electrical conductor that is formed around the drive shaft and connected to an electrical ground to provide a path of least resistance for electric charges that accumulate on the surface of the drive shaft. Some ground rings use electrically conductive wire brushes that are arranged around the drive shaft at a predetermined distance from the drive shaft surface to bleed electrical charge through the ground ring instead of through the bearing or other components in the motor.
While ground rings are useful in preventing damage to drive bearings, the charge on the drive shaft may reach a level that enables arcing of the electric charge from the drive shaft. The possibility of arcing to the ground ring makes electrical motors having ground rings unsuitable for use in some environments where the atmosphere around the motor includes potentially flammable gases or other materials that could ignite in response to arcing. For example, regulatory requirements in many countries prevent the use of electric motors in environments potentially hazardous environments where accidents or other abnormal operating conditions may expose the motor to flammable gases or other flammable materials. The regulatory standards preclude the use of existing ground rings with electric motors. In light of these deficiencies, improvements to electric motors that enable the use of electric motors in a wider range of operating environments would be beneficial.